The present invention relates to a method and apparatus for forming an excellently crystalline dielectric film with a high dielectric constant, like a CeO2 film, or ferroelectric film out of metal Ce and oxygen on an Si substrate.
In recent years, the number of C-MOS devices that can be integrated together on a single Si substrate has increased significantly because those devices have been tremendously downsized. To catch up with this trend, reduction in thickness of a gate insulating film, which is part of a MOSFET, is also in high demand. A thinner gate insulating film is needed because of the following reasons.
First of all, although the operating voltage has been reduced day after day to conserve power as much as possible, the quantity of charge required for the operation of a device remains almost the same and has not been reduced so much. Since the relationship of Q=CV (where Q is the quantity of charge, C is static electricity and V is voltage) should be met, the static electricity C that can be retained in a gate insulating film must be increased to reduce the voltage V with the quantity of charge Q kept substantially constant. The static electricity C is given by C=(∈r·S)/d, where ∈r is relative dielectric constant, S is the area of a capacitor and d is a space between electrodes. Accordingly, the static electricity C can be increased if the thickness d of a gate insulating film, which is currently made of SiO2 in many cases, is reduced. For that purpose, the gate insulating film has been thinned to a thickness between 10 and 15 nm or less than 10 nm.
However, if a gate insulating film is thinned that much, then various inconveniences might be concerned about lately; the breakdown strength of the gate insulating film might decrease or the leakage current might increase.
In view of these potential disadvantages, alternative gate insulating film materials, which have a relative dielectric constant ∈r higher than that of SiO2 and yet exhibit pretty good electrical properties comparable to those of SiO2, have been searched for. That is to say, if the relative dielectric constant ∈r is higher, then the static electricity C can be kept high even when the thickness d is increased to a certain degree. Accordingly, the required charge quantity Q is attainable even with a reduced operating voltage. Taking these points into account, methods for forming, on an Si substrate, an insulating film made of a novel insulating material with high dielectric constant and breakdown strength and low interface level and leakage current have been researched and developed to attain characteristics comparable to those of the SiO2 gate insulating film currently used.
Efforts have also been made to form an insulating film of a non-SiO2 insulator material on an Si substrate by a different type of demand. For example, an example disclosed in Japan Journal of Applied Physics 35, 4987, (1996) (which will be herein called a “first document” for convenience sake) reported research for implementing a transistor with memory function by providing a thin film with ferroelectricity for the gate of a field effect transistor. According to the technique disclosed in this document, a thin film of PbZr1-xTixO3 (PZT) with ferroelectricity, i.e., a PZT film, is formed as an exemplary thin film of that type. However, since it is difficult to form the PZT film directly on an Si substrate, an insulating film of CeO2, for example, is interposed as a buffer layer between the PZT film and the Si substrate.
Methods for forming a novel insulator film on an Si substrate to attain those characteristics, including high dielectric constant and breakdown strength and low interface level and leakage current, as in the gate insulating film mentioned above, have also been researched such that a ferroelectric or other dielectric film (e.g., superconductor film) can be formed on the Si substrate.
According to any of these suggested techniques, a CeO2 film is one of very attractive insulator materials for a buffer layer. This is because the lattice constant of CeO2 is closer to that of Si than any other known material and a lattice mismatch between CeO2 and Si is only −0.37% (i.e., aCeO2=5.411 Å and aSi=5.431 Å). In addition, since the crystal structure of CeO2 is like that of fluorite, CeO2 can form a continuous crystal lattice with the Si substrate having a diamond structure. The coordination number for all the atoms is four in Si, whereas the coordination number for oxygen atoms is four and that for Ce atoms is eight in CeO2. However, since both Si and CeO2 crystals belong to a cubic system, which is represented as a face-centered cubic lattice as a matter of principle, Si and CeO2 crystals can be stacked one upon the other by epitaxial growth (because mole ratio of oxygen to Ce is 2:1). Thus, it is possible to form a thin film with excellent crystallinity on the Si substrate, and it is easier to stack a ferroelectric or superconductor film with high crystallinity thereon. Furthermore, since the relative dielectric constant of CeO2 is as high as around 26, it is very likely that CeO2 will be used as a novel gate insulating film material in place of SiO2.
Various techniques of forming CeO2 on an Si substrate have been proposed in numerous other documents as well as the first document. Following is typical examples of them.
According to an example disclosed in Japan Journal of Applied Physics, 1765, (1993) (which will be herein called a “second document” for convenience sake), CeO2 is evaporated from a pellet-like CeO2 sintered compact by irradiating the compact with an electron beam (EB) in a molecular beam epitaxy (MBE) system including an EB evaporation unit, thereby forming an excellently crystalline CeO2 thin film on an Si substrate. In this case, decline in crystallinity of the CeO2 thin film due to the oxygen deficiency is prevented by supplying oxygen gas while CeO2 is being evaporated. In the first document, the CeO2 film is also formed by the same method.
An example disclosed in Japan Journal of Applied Physics 270, 1994 (which will be herein called a “third document” for convenience sake) uses a thin film forming technique different from that of the first and second documents. In the third document, a reactive sputterer including a target of metal Ce is used and Ce atoms are sputtered out of the target with oxygen gas supplied thereto and reacted with oxygen on the Si substrate, thereby forming an excellently crystalline CeO2 thin film on the Si substrate.
An example disclosed in Applied Physics Letters 2027, (1991) (which will be herein called a “fourth document” for convenience sake) forms a CeO2 film by a different technique from any of the techniques mentioned above. In the fourth document, an MBE system, into which ArF excimer laser radiation can be introduced externally, is used, a pellet-like CeO2 sintered compact placed inside is irradiated with the laser radiation to evaporate CeO2 therefrom and oxygen gas is introduced at the same time. In this manner, an excellently crystalline CeO2 thin film is formed on the Si substrate.
These methods of forming a crystalline CeO2 thin film as disclosed in the documents cited above, however, have the following shortcomings.
It should be noted that a family of crystallographic planes including (100), (010), (001) and so forth will be collectively referred to as a (001) plane in the following description, although such a family of planes should be labeled {001}. The same statement will be applicable to a (011) or (111) plane. Similarly, (001), (011) or (111) substrate or film will mean a substrate or film with a (001), (011) or (111) plane as its principal surface.
First, in accordance with the example disclosed in the first and second documents, oxygen and Ce are supplied at the same time by evaporating CeO2 from a pellet-like CeO2 sintered compact being heated. That is to say, since Ce and oxygen reach the surface of the Si substrate at a time, SiO2, as well as CeO2, is formed thereon. Should SiO2 be formed, the sharpness of crystallinity decreases at the interfacial structure and the planarity of the surface also deteriorates since SiO2 generally has an amorphous structure. Also, if the structure with the SiO2 film is operated as a device, a voltage applied will be concentrated on the SiO2 film with the lower dielectric constant in spite of the presence of the CeO2 film with the higher dielectric constant. As a result, it is difficult to store charge to a quantity large enough to ensure the intended function of a gate insulating film. In addition, even when such a CeO2 film mingled with SiO2 is used as a buffer layer for a ferroelectric or superconductor film, it is also difficult to apply a required voltage to the ferroelectric or superconductor layer.
Following the example disclosed in the second document, a CeO2(111) film can be formed on an Si(111) substrate. But only a CeO2(011) film, not a CeO2(001) film, can be formed on an Si(001) substrate. That is to say, even though the lattice constants of CeO2 and Si are close to each other, the plane orientations thereof are different from each other. Thus, the effects of suppressing lattice strain and preventing the generation of defects cannot be expected at all. Furthermore, although a CeO2(011) film is formed, the film actually has a polycrystalline structure, in which two types of crystals coexist on the principal surface of the Si substrate so as to be symmetrical around an axis and form an angle of 90 degrees between them. Accordingly, it is difficult to obtain a smooth and uniform single crystal thin film.
Japan Journal of Applied Physics 31, L1736, (1992) (which will be herein called a “fifth document” for convenience sake) explains the reason of this phenomenon. Specifically, it is believed that higher stability is attained where the (001) plane of Si crystals is continuous with the (011) plane of CeO2 crystals rather than where the (001) planes of these two types of crystals are continuous with each other. This is probably because dangling bonds appearing on the 2×1 streaks on the (001) plane of Si crystals that are formed in high vacuum are located close to oxygen atoms within the (011) plane of CeO2 crystals.
The fourth document shows that a CeO2(111) film with excellent crystallinity can be formed on an Si(111) substrate. In accordance with this example, an oscillation of diffraction pattern intensity (RHEED oscillation) is observed by a reflection high-energy electron diffraction (RHEED) analysis during the crystal growth. The generation of the RHEED oscillation indicates that the crystals are growing two-dimensionally, or layer by layer, while keeping high smoothness at the surface. Even when the cross section thereof is observed by TEM, the existence of large defects is hardly observable. The formation of SiO2 is not found in the interface between Si and CeO2, either. However, this document does not report on successfully forming a (001) plane of CeO2 crystals on a (001) plane of Si crystals, either.
Japan Journal of Applied Physics 29, L1199, (1990) (which will be herein referred to as a “sixth document” for convenience sake) makes a disclosure about this. Specifically, since Ce and oxygen are also supplied at a time even by using such a system, a CeO2(011) film is formed unintentionally on an Si(001) substrate.
Even in the example disclosed in the third document, a (111) plane of CeO2 with excellent crystallinity is formed on a (111) plane of an Si substrate as in the second and fourth documents. In accordance with the method disclosed in the third document, metal Ce is used as a source material. Thus, the method succeeds in supplying only Ce onto the interface of the Si substrate and suppressing the formation of SiO2. However, a layer consisting of metal Ce alone, which is needed in obtaining the excellently crystalline CeO2 film, is as thick as 5 nm. Accordingly, if the film is used as a gate insulating film for a transistor, then the operation of the transistor device is seriously affected by the existence of the thick metal layer. In addition, the third document does not report on successfully forming a CeO2(001) film on an Si(001) substrate, either. The reason thereof cannot be found in the document. But we think it would be difficult to form CeO2 crystals with the same plane orientation by relaying information about the crystal structure of the Si substrate if the metal Ce layer with the thickness of about 5 nm exists between them.